SWINGS Cruise report MD229- N/O Marion-Dufresne- Jan 11th -March 8th 2021

Introduction and objectives

SWINGS is a multidisciplinary 4-year project dedicated to elucidate trace element sources, transformations and sinks along a section crossing key areas of the Southern Ocean (SO). Major French contribution to the international GEOTRACES program (www.geotraces.org), SWINGS involves ca 80 scientists (21 international laboratories, 7 countries2). As core action of SWINGS, the SWINGS cruise (R/V
Marion-Dufresne, MD229, Geotraces section GS02) started from La Reunion on January 11th 2021 and ended at La Reunion 57 days later (March 8th, 2021). This cruise explored a large part of the South Indian Ocean (Figure 0-2) in order to tackle the following objectives:

1) establish the relative importance of sedimentary, atmospheric and hydrothermal sources of TEIs in the Indian sector of the SO
2) investigate the drivers of the internal trace element cycles: biogenic uptake, remineralization, particle fate, and export, and
3) quantify TEI transport by the Antarctic Circumpolar Current and the numerous fronts at the confluence between Indian and Atlantic Oceans.

Cruise Strategy

SWINGS strategy relies on the strong coupling between physical oceanography, biogeochemistry and modeling. During the cruise, a major and original focus has been put on the characterization of the physical, biological and chemical particle speciation in suspended and sinking particles that have been collected during SWINGS.

We realized a high spatial resolution sampling of the dissolved (<0.45 μm) and particulate (>0.45 μm) pools, from the surface to the seafloor. This harvest of data will allow a major step forward in the understanding and quantification of dissolved-particle exchanges, a major recognized bolt for the element cycle modeling. Moreover, samples to analyze dedicated tracers (e.g Th and Pa isotopes) were collected in order to better characterize the particle dynamics. Ra isotope analyses will support the quantification of land-ocean transfers while Nd ones will trace the origin of the dissolved and particulate matter. Both tracers will also help identifying and characterizing hydrothermal source occurrences. Indeed, specific attention was paid to the ocean interfaces: atmospheric and land contacts, and a segment of the South West Indian Ridge suspected to be the home of active hydrothermal sites were explored. Combined with the other suite of trace metals, we will be able to provide an estimation of the lateral and vertical transport of key trace metals from the different sources investigated along the section. We also collected samples in order to describe the taxonomic diversity of heterotrophic microbes, their metabolic potential, gene- and protein expression patterns as well as samples necessary for the enumeration of heterotrophic prokaryotes and small (up to 20 µm) autotrophic phytoplankton, 2 SWINGS PARTNERS: CNRS_UPS_LEGOS (PI, TOULOUSE), CNRS_UBO_LEMAR (PI, BREST), AMU_MIO (MARSEILLE), CNRS_UVSQ_LSCE (SACLAY), CNRS_SU_LOCEAN (PARIS), CNRS_SU_LOMIC (BANYULS), CNRS_UPS_GET (TOULOUSE), CNRS_SU_AD2M (ROSCOFF), CNRS_CECI (TOULOUSE), CSIR-SOCCO (CAPE TOWN, SOUTH AFRICA), SU-DEAS, ULB_BRUXELLES (BELGIUM), WU-SO (WASHINGTON UNIV, USA), WHOI-MBC (WOODS HOLE, USA), FU-DEOAS (FLORIDA STATE UNIV AND FLORIDA INTERNATIONAL UNIVERSITY, USA), GEOMAR (KIEL, GERMANY), PEO AND ETH (ZURICH, CH) 10 the concentration of dissolved organic carbon (DOC) and major inorganic nutrients (nitrate and nitrite, phosphate and silicic acid). Finally, dedicated biology experiments, such as nitrification, calcification or iron uptake experiments were conducted throughout the cruise.

The cruise track –at the Atlantic-Indian boundary- did cross up to 6 currents or fronts, among which the 3 majors are reported in Figure 0-2. These jets are major pathways of the general circulation, critical for chemical specie transport: our navigation strategy was regularly adapted using the SOS (Scheduler for Oceanographic Samplings), an interactive navigation tool for adaptive cruise scheduling in order to characterize these current dynamic (geostrophic calculation) as well as their trace element and isotope contents.

Jeandel C. & Planquette H., 2021. SWINGS Cruise report MD229- N/O Marion-Dufresne- Jan 11th -March 8th 2021. HAL Archives-Ouvertes.FrReport.

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